Rickettsioses represent devastating human infections. These arthropod-borne diseases are caused by obligatory intracellular bacteria of the genus Rickettsia (R.). A vaccine is not available for rickettsioses. Disseminated vascular endothelial cell (EC) infection and EC barrier dysfunction are the central pathophysiologic features of human lethal spotted fever group rickettsial (SFGR) infections. Typically, infection with SFGR is controlled by appropriate broad-spectrum antibiotic therapy if diagnosed early. Nevertheless, SFGR infections present with nonspecific signs and symptoms rendering early clinical diagnosis difficult. Untreated or misdiagnosed SFGR infections are frequently associated with severe morbidity and mortality. Comprehensive understanding of rickettsial pathogenesis is urgently needed for the development of novel prophylactics and post-infection (p.i.) therapeutics. We reported that, upon SFGR, RNase-mediated tRNA cleavage occurs and a specific subset of tRNA-derived RNA fragments (tRFs) are induced. Among them, 5'-end fragment from tRNAGlyGCC is the most prominently induced tRF, termed as tRFGlyGCC. We found that tRFGlyGCC exhibits trans-silencing activity in a sequence-specific manner and induces EC barrier dysfunction. Several lines of new evidence from our preliminary studies suggest that Exos derived from R. parkeri-infected human umbilical vein EC (HUVEC) (RCExos) at 72hr p.i. or Exos derived from plasma of 2LD50 R. parkeri-nfected mice (RMExos) on day 4 p.i. can induce dysfunction of normal recipient human brain microvascular ECs (BMECs) in a tRFGlyGCC-dependent manner. Compared with naked format, bovine serum albumin-nanoparticlized anti- tRFGlyGCC oligonucleotides (BSAanti-tRFGlyGCCs) in normal media with sera can maintain BMEC barrier functions after exposure to RCExos. These findings suggest that RCExos/RMExos-packed tRFGlyGCC may induce normal recipient EN dysfunction during SFGRs. Our goal in this proposal is to seek more experimental evidence to support our central hypothesis that targeting identified SFGR-induced tRFGlyGCC in exosomes can provide protection against SFGR by maintaining recipient EC barrier function. To test this hypothesis, we will pursue three Specific Aims: (1) biochemically corroborate that RCExos/RMExos-packed tRFGlyGCC alters the recipient EC barrier structure(s), (2) biomechanically corroborate that RCExos/RMExos-packed tRFGlyGCC causes the recipient EC barrier dysfunction, and (3) evaluate whether targeting Exos-packed tRFGlyGCC with anti-tRFGlyGCC nanoparticles can protect against lethal rickettsial infection by maintain the endothelial barrier function. We will test our hypothesis by employing cutting-edge approaches (FluidFM technology, size-exclusion chromatography, and formulation of nanoparticles for optimizing delivery of anti-tRFGlyGCC into ECs). Outcomes will provide deeper insights into the biomechanical and molecular mechanisms of SFGR infection, ultimately leading to the identification of a druggable host-dependent factor during lethal SFGR infections.